A series of Ce3+-doped (Ca,Sr)2Al2SiO7 phosphors with different Ce3+ and Ca2+/Sr2+ concentrations were prepared by a high temperature solid-state reaction technique. To get insight into the structure–luminescence relationship, the impact of incorporation of Sr2+ on structure of (Ca,Sr)2Al2SiO7 was first investigated via Rietveld refinement of high quality X-ray diffraction (XRD) data, and then the VUV–UV excitation and UV–vis emission spectra of (Ca,Sr)2Al2SiO7:Ce3+ were collected at low temperature. The results reveal that the crystal structure evolution of (Ca,Sr)2Al2SiO7:Ce3+ has influences on band gaps and Ce3+ luminescence properties including 4f–5di (i = 1–5) transition energies, radiative lifetime, emission intensity, quantum efficiency, and thermal stability. Moreover, the influence of Sr2+ content on the energy of Eu3+–O2– charge-transfer states (CTS) in (Ca,Sr)2Al2SiO7:Eu3+ was studied in order to construct vacuum referred binding energy (VRBE) schemes with the aim to further understand the luminescence properties of (Ca,Sr)2Al2SiO7:Ce3+. Finally, X-ray excited luminescence (XEL) spectra were measured to evaluate the possibility of (Ca,Sr)2Al2SiO7:Ce3+ as a scintillation material.

A series of Ba2-xEuxMgSi2O7 phosphors was prepared by a solid-state reaction method at high temperature. The theoretical density of the optimal Ba1.93Eu0.07MgSi2O7 material was calculated from the Rietveld refinement result. Eu L3-edge X-ray absorption near edge structure (XANES) spectrum was measured to confirm the valence of Eu ions in Ba2MgSi2O7. The X-ray excited radioluminescence and the thermoluminescence after β-ray irradiation were investigated based on the VUV-vis photoluminescence. The light yield of the optimal Ba1.93Eu0.07MgSi2O7 sample under X-ray excitation was estimated to be ~29,000±6000 photons/M eV. So the temperature-dependent luminescence properties of this sample under X-ray and 344 nm excitation were further studied, and the charge traps in the scintillation process were discussed through thermoluminescence spectra. The high scintillation intensity together with an appropriate intrinsic decay time and its non-hygroscopicity endow the further optimized phosphor Ba1.93Eu0.07MgSi2O7 a promising scintillation material for X-ray detection.

Herein, a new red-emitting phosphor, Na3TaF8:Mn4+, was synthesized by non-equivalent doping of Mn4+ under mild condition. Its crystal structure and morphology were characterized by the powder X-ray diffraction (XRD), scanning electron microscopy (SEM). This sample is of single phase with monoclinic crystal structure. Mn4+ might occupy the site of Ta5+ in Na3TaF8, being located at the center of uniquely tetragonal anti-prism coordinated by eight F-. The as-prepared Na3TaF8:Mn4+ has sharp red emissions with higher color purity under the blue light excitation. The luminescent behavior of Mn4+ in such host is similar with that in some hexafluorides with disordered octahedral structure. Intense red light can be observed from the single red LED based on Na3TaF8:Mn4+. These results show this phosphor to be a potentially red component for LED backlighting.Download high-res image (154KB)Download full-size image

AbstractSamples of the Ca3Sc2Si3O12 (CSS) host singly doped with Eu2+ or Yb3+, doubly doped with Eu2+ and Yb3+, and triply doped with Ce3+, Eu2+ and Yb3+ were synthesized by a sol–gel combustion process under reducing conditions. Unlike previous reports of Eu2+Yb3+ energy transfer in other systems, the energy transfer is resonant in the CSS host and the transfer efficiency reaches 100 % for lightly doped samples. The transfer mechanism is multipolar rather than electron transfer for the sample compositions employed herein. The emission intensity of Yb3+ is further enhanced by co-doping with Ce3+ in addition to Eu2+. The quantum efficiencies of the doped materials range between 9 % and 93 %.

AbstractSamples of the Ca3Sc2Si3O12 (CSS) host singly doped with Eu2+ or Yb3+, doubly doped with Eu2+ and Yb3+, and triply doped with Ce3+, Eu2+ and Yb3+ were synthesized by a sol–gel combustion process under reducing conditions. Unlike previous reports of Eu2+Yb3+ energy transfer in other systems, the energy transfer is resonant in the CSS host and the transfer efficiency reaches 100 % for lightly doped samples. The transfer mechanism is multipolar rather than electron transfer for the sample compositions employed herein. The emission intensity of Yb3+ is further enhanced by co-doping with Ce3+ in addition to Eu2+. The quantum efficiencies of the doped materials range between 9 % and 93 %.

•Site occupation of Ce3+ in Sr4Ca4La2(PO4)6O2 was studied by experiments.•Three main types of Ce centers were identified based on low temperature spectra.•The Ce centers on La(6 h) sites was dominant in number supported by calculations.Cerium-doped oxyapatite phosphors, Sr4Ca4La2(PO4)6O2: Ce3+, are prepared by a traditional solid-state reaction method. X-ray diffraction (XRD) refinement reveals that the hexagonal Sr4Ca4La2(PO4)6O2 structure is characterized by a random distribution of Sr and Ca atoms on the nine-coordinated cationic 4f sites and of Sr, Ca, and La atoms on the seven-coordinated cationic 6 h sites. Photoluminescence properties of Ce-doped samples are then studied with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperature. Three main types of occupation sites for Ce3+ are identified based on analysis of emission and excitation spectra and of luminescence decay behaviors. The Ce3+ occupation on the seven-coordinated La (6 h) site is found to be dominant, which is supported by wave function-based CASSCF/CASPT2 embedded cluster calculations on Ce3+ 4f → 5d transition energies at the spin-orbit level. The role of the coordinated oxygen ion that is not bonded with P5+ in the 5d centroid shift of CeLa(6h)3+ is emphasized. The thermal stability and doping concentration dependence of the 5d luminescence are also investigated and discussed in association with the coordination structures of Ce3+.

In this work, we demonstrate a potential thermometric material after systematic studies on the concentration/temperature-dependent spectroscopic properties of Pr3+ excited multiplets and of the Pr3+–Ti4+ intervalence charge transfer (IVCT) state in (La1−xPrx)2MgTiO6. The experimental results indicate that the electron population efficiency between the involved Pr3+ 4f multiplets is directly governed by multi-phonon relaxation (MPR) and cross relaxation (CR), and the IVCT state provides an additional contribution to the 1D2 luminescence. A schematic energy level diagram is proposed to illustrate the electron population pathway in Pr3+ doped La2MgTiO6. The observations clarify that the dramatic thermal-quenching of 3P0 luminescence is mainly induced by the electronic configuration crossover between the 3P0 multiplet and the IVCT state. On the other hand, the 1D2 luminescence possesses an excellent thermal stability in a large temperature region. These temperature sensing features of the Pr3+ doped La2MgTiO6 material indicate its potential application in optical thermometric techniques.

A series of LuNbO4:xDy3+ (x = 0–0.20) phosphors was prepared using a high-temperature solid-state reaction technique. X-ray diffraction (XRD) along with Rietveld refinement, field emission scanning electron microscopy (FE-SEM) observations, diffuse reflectance spectra (DRS), UV-vis photoluminescence (PL), fluorescence decays, PL quantum yields (QYs), and low-voltage cathodoluminescence (CL) were employed to characterize the phosphors. Nonradiative relaxation and host sensitization dramatically influence the LuNbO4:Dy3+ luminescence spectra and decay dynamics. It is shown that cross-relaxation arising from electric dipole–dipole interactions between adjacent Dy3+ ions is the leading mechanism of quenching the Dy3+ emission. The host sensitization for Dy3+ emission in LuNbO4 was confirmed and the energy transfer efficiency from the host to Dy3+ increased with increasing Dy3+ doping concentration/temperature. Upon excitation with ultraviolet light (261 nm) and a low-voltage electron beam (2 kV, 127 μA cm−2), the synthesized LuNbO4:Dy3+ phosphors show both the blue broadband emission of the LuNbO4 host and the characteristic emission of Dy3+ (the dominant one is the 4F9/2 → 6H13/2 transition, yellow), and the luminescence colour of the LuNbO4:Dy3+ phosphors can be tuned over a large gamut of colours by varying the Dy3+ doping concentration, and a single-phase intense white-light-emission has been achieved in the LuNbO4:0.015Dy3+ phosphor. On the basis of the good UV-vis PL and CL properties, LuNbO4:Dy3+ phosphors might be promising for applications in UV light-emitting diodes (UV-LEDs) and field emission displays (FEDs).

•Red phosphors Rb2TiF6:Mn4+ have been synthesized by the ion exchange method.•This red phosphor exhibits intense red emission under blue light excitation.•Rb2TiF6:Mn4+ shares excellently thermal quenching resistance and color stability.•The white LEDs based on this phosphor exhibit excellently optical properties.Red-emitting phosphor plays a critical role in improving performance of the phosphor-converted white light-emitting diodes (pc-WLEDs). Herein, a red-emitting phosphor, Rb2TiF6:Mn4+, was synthesized via the ion exchange method under mild condition. The crystal structure and morphology were characterized by the powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The Rietveld refinements of Rb2TiF6:Mn4+ indicate that this sample is of single phase with hexagonal crystal structure. The as-prepared Rb2TiF6:Mn4+ has sharp red emissions with broad excitation band at ∼460 nm. The luminescent behavior of Mn4+ was discussed in detail. The temperature-dependent emission spectra of Rb2TiF6:Mn4+ indicate that this phosphor shares high thermal quenching resistance and excellent color stability. A series of WLEDs with tunable color rendering index and color temperature were fabricated by combining commercial Y3Al5O12:Ce3+ and Rb2TiF6:Mn4+ on blue GaN-LED chips. With the addition of Rb2TiF6:Mn4+, WLED with wide gamut was obtained with low color temperature (3123 K), high color rendering index (91.5) and high luminous efficacy (187.9 lm/W). These findings show this phosphor could be a promising commercial red phosphor in wide color-gamut WLEDs.Download high-res image (134KB)Download full-size image

The energy transfer between Ce3+ and Eu2+ has been investigated in the host Ca3Sc2Si3O12 (CSS), prepared by a modified sol–gel method. Excitation and emission measurements from the near-infrared to the vacuum ultraviolet spectral regions have been performed upon CSS, Ce3+-doped CSS, Eu2+-doped CSS and Ce3+, Eu2+-co-doped CSS, at various concentrations, including experiments at temperatures range of 15–460 K. The energy transfer efficiency from Ce3+ to Eu2+ can approach 90%, and the Ce3+ donor decay curves for different Eu2+ acceptor concentrations in the codoped system were fitted by the Inokuti–Hirayama method, indicating that it is energy transfer induced by electric dipole interaction. The use of the Ce3+, Eu2+ couple in the CSS host as a wideband harvester with an emission profile tailored to the response of the silicon solar cell in solar energy conversion suffers from two main drawbacks relating to valence instability and emission quenching of Eu2+. Possible solutions are suggested.

Ce3+, Eu3+, and Tb3+ singly doped, Ce3+-Tb3+, Tb3+-Eu3+, and Ce3+-Eu3+ doubly doped, as well as Ce3+-Tb3+-Eu3+ triply doped LiYSiO4 phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed to determine the structure of host compound. The cross-relaxation (CR) of Tb3+ is quantitatively analyzed with the Inokuti–Hirayama model of energy transfer (ET), and the site occupancy is confirmed by emission spectra of Eu3+. ET and metal–metal charge transfer (MMCT) are systematically investigated in Ce3+-Tb3+, Tb3+-Eu3+, and Ce3+-Eu3+ doubly doped systems. The combined effects of ET and MMCT on luminescence and emission color of Ce3+-Tb3+-Eu3+ triply doped samples are discussed in detail, showing that the photoluminescence emission is tunable in a large color gamut.

In this work, we report the tunable emission properties of Ce3+ in an apatite-type LiY9(SiO4)6O2 compound via adjusting the doping concentration or temperature. The occupancies of Ce3+ ions at two different sites (Wyckoff 6h and 4f sites) in LiY9(SiO4)6O2 have been determined by Rietveld refinements. Two kinds of Ce3+ f–d transitions have been studied in detail and then assigned to certain sites. The effects of temperature and doping concentration on Ce3+ luminescence properties have been systematically investigated. It is found that the Ce3+ ions prefer occupying Wyckoff 6h sites and the energy transfer between Ce3+ at two sites becomes more efficient with an increase in doping concentration. In addition, the charge-transfer vibronic exciton (CTVE) induced by the existence of free oxygen ion plays an important role in the thermal quenching of Ce3+ at 6h sites. Because of the tunable emissions from cyan to blue with increasing temperature, the phosphors LiY9(SiO4)6O2:Ce3+ are endowed with possible thermometric applications.

Ce3+, Pr3+ singly doped and Ce3+–Pr3+ doubly doped LaBSiO5 phosphors were prepared by a high-temperature solid state reaction route. The synchrotron radiation VUV–UV excitation spectra of Ce3+ and Pr3+ were measured and the luminescence properties were discussed. The band gap of host compound LaBSiO5 was estimated from the VUV excitation spectrum of sample activated with Gd3+. Furthermore, the influence of temperature on luminescence of Ce3+ was discussed and the activation energy for thermal quenching was evaluated. The occurrence of energy transfer from excited 5d states of Pr3+ to Ce3+ was confirmed by the emission spectra of co-doped samples La0.99−xPrxCe0.01BSiO5 and La0.995−xPr0.005CexBSiO5 with different x values.

The lithium yttrium silicate series of LiY1–xLnxSiO4 exhibits superb chemical and optical properties, and with Ln = Ce3+, Sm3+, its spectroscopic characteristics and luminescence dynamics are investigated in the present work. Energy transfer and nonradiative relaxation dramatically influence the Ln3+ luminescence spectra and decay dynamics, especially in the Ce3+–Sm3+ codoped phosphors. It is shown that thermal-quenching of the blue Ce3+ luminescence is primarily due to thermal ionization in the 5d excited states rather than multiphonon relaxation, whereas cross-relaxation arising from electric dipole–dipole interaction between adjacent Sm3+ ions is the leading mechanism that quenches the red Sm3+ luminescence. In the codoped systems, Ce3+–Sm3+ energy transfer in competing with the thermal quenching enhance the emission from Sm3+. The combined influences of concentration quenching, thermal ionization, and energy transfer including cross-relaxation on the luminescence intensity of single-center and codoped phosphors are analyzed based on the theories of ion–ion and ion–lattice interactions.

A series of LuNbO4:Pr3+ phosphors was prepared by a solid-state reaction method at high-temperature. Rietveld refinements were performed based on powder X-ray diffraction (XRD) data. Diffuse reflectance spectra (DRS), UV–vis photoluminescence (PL), time-resolved emission spectra (TRES), and fluorescence decays were utilized to study the luminescence and host sensitization processes of Pr3+ in LuNbO4. Excitation wavelength dependent luminescence of LuNbO4:Pr3+ was investigated and explained in consideration of the processes of nonradiation relaxation via cross-relaxation, multiphonon relaxation, and crossover to the intervalence charge transfer (IVCT) state. Furthermore, the host sensitization of Pr3+ emission in LuNbO4 was confirmed and the energy transfer efficiency from host to Pr3+ increased with increasing Pr3+ doping concentration/temperature. Because the change of emission intensities for both blue from the host and red from 1D2 is sensitive to temperature, a large variation of emission color is observed between RT and 500 K.

Eu3+, with the 4f6 electronic configuration, generally exhibits bright red f-f emissions arising from its 5D0 multiplet, and Eu3+ doped phosphors have attracted lots of attention for applications in lighting and display fields. However, the electron population mechanisms between relevant Eu3+ excited states as well as charge-transfer state (CTS) still need to be further clarified since the puzzles on these issues limit the exploration of new luminescent materials and the improvement of the luminescence efficiency of the potential phosphors. In this work, a series of Sr0.99[La(1–x)Eux]1.01Zn0.99O3.495 phosphors was prepared by a high-temperature solid-state reaction technique and was characterized by X-ray diffraction (XRD) measurements at different temperatures, infrared (IR) spectrum, and diffusion reflectance spectra (DRS) at room temperature (RT). The temperature-, doping concentration-, and excitation wavelength-dependent luminescence properties were systematically studied to clarify the population pathway, de-excitation mechanism, and decay dynamics of Eu3+ in this low-phonon-frequency compound. The impacts of cross relaxation (CR) and multiphonon relaxation (MPR) processes on the luminescence and decay spectra were investigated in detail. The special coordination polyhedron around Eu3+ played a dominant role in the intense Eu3+ 5D0–7F4 emission. The CTS peaks shifted to longer wavelengths with increasing temperatures, which seems to relate to the lattice expansion at higher temperatures.

A series of Sr1–2xCexNaxB2Si2O8 and Ba1–2xCexKxB2Si2O8 (x = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) samples were synthesized by a high-temperature solid-state reaction. The low temperature excitation, emission, and fluorescence decay spectra together demonstrated that all spectral bands arise from the Ce3+ ions located at only one kind of lattice site. The first-principles calculations of the structural and electronic properties of pure and Ce3+-doped MB2Si2O8 (M = Sr, Ba) were performed, and the obtained results were used for understanding the structural changes after doping and identification of the observed position of the host absorption bands. The measured 4f-5d excitation and emission spectra of Ce3+ ions doped in MB2Si2O8 were analyzed and simulated in the framework of the crystal-field (CF) theory. The electron–phonon coupling effect generally ignored in most studies published so far was also taken into account by applying the configurational coordinate model. The validity of such a combined insight into the 5d CF energy level positions and the Stokes shift has been confirmed by analyzing the dependence of the Ce3+ spectroscopic properties on the dopant concentration. In addition, the influence of temperature on the luminescent properties of the studied samples was also explored and is discussed.

The downshifting from Ce3+ blue emission to Yb3+ near-infrared emission has been studied in the garnet host Ca2.8–2xCe0.1YbxNa0.1+xSc2Si3O12 (x = 0–0.36). The downshifting does not involve quantum cutting, but one incident blue photon is transferred from Ce3+ to Yb3+ with an energy transfer efficiency up to 90% when x = 0.36 for the Yb3+ dopant ion. For x ≤ 0.15, a multiphonon-assisted electric dipole–electric quadrupole mechanism of energy transfer dominates, while for the highest concentration of Yb3+ employed, the electron transfer mechanism is confirmed. A temperature-dependent increase of the Ce3+ → Yb3+ energy transfer rate does not exclusively indicate the electron transfer mechanism. The application of the material to solar energy conversion is indicated.

A series of LiCaBO3:Ce3+/Pr3+ phosphors were prepared by a high-temperature solid state reaction method. A structure refinement for the LiCaBO3 compound was performed based on powder X-ray diffraction (XRD) data. The energies of the crystal field split excited 5d states of Ce3+ and Pr3+ in LiCaBO3 were determined from synchrotron radiation VUV-UV excitation spectra. Furthermore, the influence of the doping concentration and temperature on the emission properties of Ce3+ was investigated, and the vacuum referred binding energy (VRBE) scheme for all lanthanide 4f and 5d states in LiCaBO3 was constructed to estimate the thermal activation energy of Ce3+ luminescence quenching. The Pr3+ to Ce3+ energy transfer (ET) and its influence on luminescence decays of Pr3+ and Ce3+ were studied in detail. Finally, the X-ray excited luminescence spectra were measured to evaluate the possible X-ray detection applications.

0.5% Ce3+ doped BaCa2MgSi2O8 phosphor was prepared by a conventional solid state reaction method. Luminescence spectra as well as fluorescence decay were monitored in the VUV-UV range. Ce3+ emissions are assigned to cerium ions on a Ba2+ site, and the five 4f–5d excitation bands of Ce3+ were determined at low temperature. The light yield is estimated to be around 10600 ph MeV−1 under X-ray excitation. X-ray absorption near-edge structure (XANES) was explored to study the energy transfer efficiency to optical centers from each element in the phosphor; the results show that the contributions to luminescence are not identical for each element.

•Nanorods LaPO4 with different sizes are prepared with hydrothermal method.•Luminescence properties of bulk and nano LaPO4:Tb samples are compared.•La, Tb and P contribute to luminescence positively at the measured absorption edges.•At oxygen absorption edges, quantum efficiency of optical emission decreases.Tb3+ activated LaPO4 nano-phosphors have been prepared by a hydrothermal method. Luminescence spectra in VUV–vis range as well as fluorescence decays were studied for bulk and nano-phosphors. Bulk sample exhibits a relatively fast decay time. The 5D3 emissions from Tb3+ ions increase under X-ray excitation in comparison with that under ultraviolet light excitation. X-ray absorption near-edge structure (XANES) was employed to study the chemical environment and energy transfer efficiency to optical emission. XANES results across different element absorption edges indicate that the chemical environment does not change significantly, only oxygen contributes to luminescence negatively.

•The main EVI parameters for Ce3+ in GdAl3(BO3)4 were simulated.•The VRBE scheme was constructed, thermal-quenching of Ce3+ luminescence was predicted.•Multi-step energy transfer processes occur under different excitations.Ce3+ and/or Tb3+ doped GdAl3(BO3)4 phosphors were prepared via a high-temperature solid-state reaction method. The band gap of GdAl3(BO3)4 is determined from the VUV excitation spectrum of a Ce3+ doped sample at 14 K and further confirmed by that of undoped and Tb3+ doped samples at RT. X-ray excited luminescence (XEL) of GdAl3(BO3)4: Ce3+ was measured. The main electron-vibrational interaction (EVI) parameters for Ce3+ in GdAl3(BO3)4 were simulated. The vacuum referred binding energy (VRBE) scheme of lanthanide 4f/5d states in GdAl3(BO3)4 was constructed. Using this scheme, the lowest 5d excitation band of Pr3+ in GdAl3(BO3)4 was predicted, showing that the estimation is in agreement with the experimental result. The occurrence of multi-step energy transfer processes which include Gd3+→Ce3+, Gd3+→Tb3+, Ce3+→Tb3+, and Tb3+→Gd3+, Tb3+→Ce3+ under different excitations was derived from excitation, emission, and fluorescence decay spectra.

•Site occupancy of Ce3+ in β-Ca2SiO4 was studied by spectral and ab initio methods.•Two distinct Ce3+ sites were revealed by experimental spectral measurements.•The two Ce3+ sites were identified as Ce3+ on Ca1 and Ca2 sites of β-Ca2SiO4.•The Ce3+ ions prefer to occupy Ca2 sites over Ca1 sites in β-Ca2SiO4.•The change of the lowest 4f→5d transition energy from CeCa1 to CeCa2 was analyzed.Low-temperature photoluminescence properties of the β-Ca2(1−x)CexNaxSiO4 (x = 0.0005) phosphor synthesized by a solid-state reaction method are investigated with excitation energies in the vacuum ultraviolet (VUV) to ultraviolet (UV) range. Two distinct types of emission and excitation spectra are observed, which are attributed to 4f–5d transitions of two different sets of Ce3+ centers. On the basis of density functional theory (DFT) calculations within the supercell model and wave function-based CASSCF/CASPT2 embedded cluster calculations, the two sets of Ce3+ centers are ascribed to the Ce3+ located on the seven-coordinated Ca1 and eight-coordinated Ca2 sites, respectively. Furthermore, from the observed relative spectral intensities, DFT total energy calculations, and comparison of experimental and calculated 4f → 5d transition energies, it is concluded that the occupation of Ce3+ on the Ca2 site is more energetically favorable than the occupation on the Ca1 site. Finally, the redshift of the lowest 4f → 5d transition of Ce3+ on the Ca2 site relative to that on the Ca1 site is discussed in terms of the changes of the 5d centroid energy and crystal-field splitting with the local coordination structure.

•The VUV–vis PL and low-voltage CL of Ba2MgSi2O7:Mn2+ were investigated in detail.•The main EVI parameters of Mn2+ in Ba2MgSi2O7 were evaluated.•Further optimized Ba2Mg0.95Mn0.05Si2O7 sample shows potential applications in FEDs.A series of Mn2+ doped Ba2MgSi2O7 phosphors with a nominal formula Ba2Mg1−xMnxSi2O7 (x = 0.01–0.13) were prepared by a high-temperature solid-state reaction technique. Their VUV–vis photoluminescence spectra, fluorescence decay curves, and low-voltage cathodoluminescence spectra were measured and discussed in detail. The results confirm that the Mn2+ ions enter into the four-coordinated Mg2+ site in Ba2MgSi2O7 host lattice. The main electron-vibrational interaction (EVI) parameters (such as Huang–Rhys factor, effective phonon energy and position of the zero-phonon line) were evaluated from the experimental spectra. Furthermore, the concentration and the temperature dependence of the emission intensity and the luminescence decays of Ba2MgSi2O7:Mn2+ were investigated, accordingly the schematic energy levels for Mn2+ in Ba2MgSi2O7 were proposed. Due to the high emission intensity under low-voltage electron beam excitation, a further optimized sample Ba2Mg0.95Mn0.05Si2O7 can be considered as a promising green phosphor for field emission displays (FEDs).

•An orange-red emission was observed in GdAl3(BO3)4:Sm3+ under VUV excitation.•Photoluminescence and the fluorescence decays verify Gd3+ → Sm3+ energy transfer.•The energy transfer process was proposed.•VUV-excited emission of Eu3+-doped and Sm3+-doped GdAl3(BO3)4 was compared.In order to develop a new warm-color emission phosphor under vacuum ultraviolet (VUV) excitation, trivalent samarium ion (Sm3+) doped GdAl3(BO3)4 was prepared by a solid state reaction technique at high temperature. The VUV excitation and emission spectra of as-synthesized sample GdAl3(BO3)4:Sm3+ were determined in the Beijing Synchrotron Radiation Facilities (BSRF). Compared with the emission of GdAl3(BO3)4:Eu3+, GdAl3(BO3)4:Sm3+ shows an orange-red emission under Xe 172 nm VUV excitation, which indicates that Sm3+ ion can be a possible activator to obtain warm color emission in the field of lighting. Furthermore, for improving the emission of Sm3+ doped in GdAl3(BO3)4, the energy transfer process from Gd3+ in the host to the activator Sm3+ was investigated through the analysis of spectroscopic characteristics and luminescence decay curves of GdAl3(BO3)4:Sm3+. The results reveal that there exists energy transfer from Gd3+ to Sm3+, but the energy transfer from Sm3+ to Gd3+ is inefficient.A new warm-color emission phosphor GdAl3(BO3)4:Sm3+ under Xe 172 nm VUV excitation was developed. Compared with the emission of GdAl3(BO3)4:Eu3+, GdAl3(BO3)4:Sm3+ show an orange-red emission. For improving the emission of Sm3+ doped in GdAl3(BO3)4, the energy transfer process from Gd3+ to Sm3+ was investigated.

Low-temperature photoluminescence properties of Sr1–2xCexNaxAl2O4 (x = 0.001) synthesized by a solid-state reaction method are measured with excitation energies in the vacuum ultraviolet (VUV) to ultraviolet (UV) range. Two distinct activator centers with different emission and excitation intensities are observed and attributed to Ce3+ occupying the Sr1 and Sr2 sites of SrAl2O4 with different probabilities. Hybrid density functional theory (DFT) calculations within the supercell model are then carried out to optimize the local structures of Ce3+ located at the two Sr sites of SrAl2O4, on which wave function-based CASSCF/CASPT2 embedded cluster calculations with the spin–orbit effect are performed to derive the Ce3+ 4f1 and 5d1 energy levels. On the basis of the observed relative spectral intensities, the calculated DFT total energies, and the comparison between experimental and calculated 4f → 5d transition energies, we conclude that, in SrAl2O4:Ce3+, the dopant Ce3+ prefers to occupy the slightly smaller Sr2 site, rather than the larger Sr1 site as proposed earlier. Furthermore, by using an established linear relationship between the lowest 4f → 5d transition energies of Ce3+ and Eu2+ located at the same site of a given compound, we find that, in SrAl2O4:Eu2+, the dominant green emission observed at room temperature arises from Eu2+ located at the Sr2 site of SrAl2O4.

Photoluminescence properties of Ce-doped Sr3Al2O5Cl2 crystals prepared by a solid-state reaction method are first investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range. Six bands are observed in the excitation spectrum of the Ce3+ 5d → 4f emission at 15 K. The highest energy band is attributed to the host excitonic absorption, from which the band gap energy of the host is estimated to be around 7.2 eV. The four lowest energy bands are assigned to the 4f1 → 5d1–4 transitions of Ce3+ located on the three distinct Sr2+ sites in Sr3Al2O5Cl2 with almost equal probability, based on a comparison between excitation band maxima energies and 4f → 5d transition energies obtained from wave-function-based CASSCF/CASPT2 calculations with spin–orbit coupling on Ce-centered embedded clusters. The 4f1 → 5d5 transition, not observed in the low-temperature excitation spectrum, is found to be overshadowed by a nearby defect-related excitonic absorption. On the basis of present experimental and calculated results for Ce-doped Sr3Al2O5Cl2, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Sr2+ sites of Sr3Al2O5Cl2 is constructed and discussed in association with experimental findings.

In this work, we first investigate the relationship between temperature and lattice parameters by means of Rietveld refinement and then demonstrate its impact on the luminescence peak position of Eu2+ in Sr8(Si4O12)Cl8. It is found that with increases in temperature, lattice expansion takes place without significant distortion of the coordination around Eu2+. As a result, the crystal field splitting of the Eu2+ 5d state decreases. At the same time, with the experimental data of the full width at half-maximum of Eu2+ emission at different temperatures and the infrared spectrum, the effective phonon frequency is evaluated and the main vibration motions are determined using first-principles calculation. Due to the high light yield under X-ray excitation and the excellent thermal stability of luminescence intensity and decay, a further optimized sample Sr7.7Eu0.3(Si4O12)Cl8 could be a potential scintillation material.

Photoluminescence properties of Ba2MgSi2O7:Eu2+ synthesized by a solid-state reaction method are first investigated in the vacuum ultraviolet (VUV) to visible (vis) excitation energy range. The band gap of the host is found to be around 7.44 eV. The incorporation of Eu2+ leads to bright green luminescence with weak thermal quenching above room temperature. Cathodoluminescence (CL) properties under low-voltage excitations are then studied, and the results suggest a potential application of the compound in field emission displays (FEDs). Electronic properties of the compound are finally calculated by a hybrid density functional theory (DFT) method, and are discussed in association with observed luminescence properties.

•NaCaPO4: Eu2+ exhibits a green emission and resists to the current saturation with low-voltage electron beam excitation.•Under low-voltage electron beam excitation, NaCaPO4: Tb3+ emits a color-tunable light resulted from cross-relaxation process.•Eu/Tb L3-edge XANES spectra confirmed the occurrence of Eu3+ and inexistence of Tb4+ ions in all samples.Eu2+/Tb3+ single-doped and co-doped NaCaPO4 phosphors were synthesized by a high temperature solid-state reaction method. Their photoluminescence, low-voltage cathodoluminescence and Eu/Tb L3-edge X-ray absorption near edge structure (XANES) were studied. Under low-voltage electron beam excitation, NaCaPO4: Eu2+ exhibits a green emission and NaCaPO4: Tb3+ emits a color-tunable light resulted from cross-relaxation process. For co-doped samples NaCaPO4: Eu2+, Tb3+, dominant emission is from f–d transition of Eu2+ and the 5D4–7F5 transition of Tb3+ is overlapped to the former one. Eu/Tb L3-edge XANES spectra confirmed the occurrence of Eu3+ and inexistence of Tb4+ ions in the samples.

•Phosphors have relative better CIE chromaticity coordinates.•Rietveld structural refinement shows two different sites for Eu3+ ions.•Phosphors exhibit high quenching concentration.•Critical transfer distance was calculated.Red-emitting phosphors Ba2Gd8(SiO4)6O2:Eu3+ (BGS:Eu3+) with silicate apatite structure were prepared by the conventional high temperature solid state reaction method. Rietveld structural refinement based on X-ray diffraction data indicated that there are two different sites for Eu3+ occupying in the host. It was found that the phosphors BGS:Eu3+ exhibit red emission with high quenching concentration at ~70.75 at%, then the critical transfer distance of Eu3+ in BGS:Eu3+ was calculated to be ~12.3 Å. More importantly, it has better CIE chromaticity coordinate for white light-emitting diode (w-LED) application in comparison with the commercial phosphor (Y,Gd)BO3:Eu3+ (YGB:Eu3+) under near ultraviolet (n-UV) 393 nm excitation.Strong red-emitting phosphor Ba2Gd8(SiO4)6O2:Eu3+ (BGS:Eu3+) has been obtained and this phosphor showed high quenching concentration and better CIE chromaticity coordinate for lighting and display applications.

Series of NaLa(PO3)4:Tb3+/Eu3+ phosphors were prepared by a high-temperature solid-state reaction technique. Structure refinements were performed based on powder X-ray diffraction (XRD) data. VUV–UV–vis photoluminescence (PL), fluorescence decays, time-resolved emission spectra (TRES), and low-voltage cathodoluminescence (CL) spectra were utilized to investigate the luminescence and energy transfer processes. Under VUV–UV light and low-voltage electron beam excitation, NaLa(PO3)4:Tb3+ and NaLa(PO3)4:Eu3+ exhibit characteristic emissions of Tb3+ (5D4 → 7FJ) and Eu3+ (5D0 → 7FJ), respectively. By adjusting the doping concentration of Eu3+ ions in NaTb0.70La(0.30-x)Eux(PO3)4, tunable emission colors are realized in a large color gamut, in which energy transfer from Tb3+ to Eu3+ was observed and discussed in detail. On the basis of the good VUV–vis PL and CL properties, NaLa(PO3)4:Tb3+/Eu3+ phosphors might be promising for applications in plasma display panels (PDPs) and field emission displays (FEDs).

Eu2+/Tb3+ single-doped and codoped NaCaPO4 phosphors were prepared by a high-temperature solid-state reaction method. Their VUV–UV excitation spectra, emission spectra, fluorescence decay spectra, X-ray excited luminescence (XEL), and cathodoluminescence (CL) spectra were investigated and discussed. The main parameters of the electron–vibrational coupling between the Eu2+ 5d states and host lattice vibrations (such as Huang–Rhys factor, effective phonon energy, and position of the zero-phonon line of the 4f65d–4f7 emission of Eu2+) were estimated from the experimental spectra. For codoped samples NaCaPO4:Eu2+, Tb3+, enhanced green emission of Eu2+ by energy transfer (ET) from the 5D3 level of Tb3+ was found.

The local structures and 4f → 5d transition energies of Ce3+ located on the two crystallographic strontium sites of Sr3AlO4F, with charge compensation by means of nearby sodium substitutions for strontium (NaSr′) or oxygen substitutions for coordinating fluorine (OF′), have been studied using the density functional theory (DFT) within the supercell model and the wave function-based embedded cluster calculations, respectively. The DFT total energy calculations show that Ce3+ prefers strongly to occupy the eight-coordinated (Sr2) site over the ten-coordinate (Sr1) site. On the basis of the results from embedded cluster calculations at the CASPT2 level with the spin–orbit effect, the experimentally observed excitation bands are identified in association with the charge-compensated cerium centers. Especially, the two bands observed at ∼404 and ∼440 nm have been both assigned to the Ce3+ located at the Sr2 sites but with compensation by one and two nearest-neighbor OF′ substitutions, respectively, rather than to the Ce3+ on the Sr1 and the Sr2 sites, respectively, as proposed earlier. Furthermore, the structural and electronic reasons for the red shift of the lowest 4f → 5d transition caused by coordinating OF′ substitutions are analyzed in terms of the variations in centroid energy and crystal-field splitting of the 5d1 configuration with the local environment. Finally, the thermal quenching of 5d luminescence at relatively high Ce3+ concentrations is discussed on the basis of the electronic properties calculated with the hybrid DFT method.

Ultraviolet and vacuum ultraviolet spectra are reported for Ce3+ and Eu2+ singly doped into the Sr5(PO4)3Cl lattice, synthesized by a solid-state process. The maxima of the emission intensities are at 346 nm (Ce3+) and 442 nm (Eu2+), with lifetimes of 25 ns and 494 ns, respectively. The Ce3+ excitation spectra exhibit five bands assigned to vibronic structures of the 4f1 → 5d1 electronic transition, in addition to the host absorption at 166 nm at 295 K. The 5d centroid shift is similar to that in SrCl2:Ce3+. The Eu2+ excitation spectrum is mainly comprised of two 4f7 → 4f65d broad bands between 200 nm and 450 nm, peaking at 343 nm and 275 nm. Both of the singly doped systems exhibit concentration quenching, with energy transfer rates being in the low (μs)−1 range for concentrations up to 4 at.% of total cations. The energy transfer rates are linearly and quadratically related to dopant ion concentrations of Ce3+ and Eu2+, respectively. The energy transfer between Ce3+ and Eu2+ in the co-doped Sr5(PO4)3Cl lattice has been studied by the analysis of intensity and decay measurements of both Ce3+ and Eu2+ emissions. The electric dipole–electric dipole transfer has an efficiency of 91% in Sr4.8−xCe0.01EuxNa0.01(PO4)3Cl, with a critical distance of 21 Å. Although energy transfer between Ce3+ and Eu2+ upon excitation into the overlapping Ce3+/Eu2+ absorption band is definitely demonstrated by the dramatic shortening of Ce3+ lifetime, the emission of Eu2+ is also enhanced at low Ce3+ concentrations using exclusive excitation into the Eu2+ absorption band. The quantum yields (QY) of the co-doped system approach those of BaMgAl10O17:Eu2+ (BAM) for the excitation wavelengths of 317 and 365 nm, but are inferior for 254 nm excitation. The addition of 1 at.% Ce3+ to Sr4.98Eu0.02(PO4)3Cl increases the QY by ∼20% of the original value. The cathodoluminesce of the co-doped phosphor is comparable to that of BAM, with CIE coordinates of the emission (0.274, 0.237), so that the application as a field emission display phosphor is proposed.

BaCa2MgSi2O8:Eu2+ phosphors have been prepared by a conventional high temperature solid state reaction technique. The emission and excitation spectra as well as the luminescence decays were investigated, showing that Eu2+ ions enter both Ba2+ and Ca2+ sites in the host. The structural refinement reveals that about 70% Eu2+ ions occupy Ba2+ sites in samples Ba1−xEuxCa2MgSi2O8 and BaCa2(1−x)Eu2xMgSi2O8. By investigation of the thermal-quenching, the schematic energy levels for Eu2+ in BaCa2MgSi2O8 were proposed. Intense blue emission was observed under 147, 172 and 254 nm excitation in comparison with the commercial blue phosphor BaMgAl10O17:Eu2+ (BAM), demonstrating the potential application of the phosphors in plasma display panels (PDPs) and tri-color fluorescent tubes.

The potential application of Sr8(Si4O12)Cl8:Eu2+ phosphor in wide gamut 3D-PDP and 3D-FED is demonstrated in consideration of its high emission intensity (∼148% and ∼280% of commercial BAM) under VUV light and low-voltage electron beam excitation, suitable chromaticity coordinates (0.136, 0.298) and short decay time (∼0.9 μs).

Eu3+-activated La2CaB10O19 phosphors with nominal chemical formulae La2−xEuxCaB10O19 and La2Ca1−2xEuxNaxB10O19 were prepared. The luminescence properties under VUV-UV excitation were investigated. Luminescence spectra reveal Eu3+ ions occupy both Ca2+ and La3+ sites in all samples. The O2− → Eu3+ charge transfer bands (CTBs) and the 5D0 → 7F0 emissions of Eu3+ at Ca 2+ site and La3+ site are identified, respectively. The schematic energy levels for Eu3+ in two sites were achieved. Luminescence decays of the 5D0 → 7F0 transitions were measured for both sites.

•The five 4f→5d excitation bands of Ce3+ in LaO8 sites of NaLa(PO3)4 are reported.•The lowest 4f→5d excitation band of Pr3+ is verified.•Pr3+→Ce3+ energy transfer occurs in Ce3+/Pr3+ codoped samples.A series of NaLa1−xCex(PO3)4, NaLa1−yPry(PO3)4 and NaLa0.7−zCe0.3Prz(PO3)4 phosphors with different doping concentrations (x, y and z values) are prepared through a solid-state reaction route. Five 4f→5d excitation bands of Ce3+ in NaLa(PO3)4 are identified according to the VUV-UV excitation spectra. Influence of doping concentration on the emission spectrum of NaLa1−xCex(PO3)4 is discussed. The lowest 4f→5d excitation band of Pr3+ is verified through the comparison of excitation spectra of Ce3+ and Pr3+. The fluorescence decays of NaLa1−yPry(PO3)4 are studied. Due to the effective spectral overlap between the emission of Pr3+ and the excitation of Ce3+, the Pr3+→Ce3+ energy transfer in NaLa0.7−zCe0.3Prz(PO3)4 phosphors are investigated.

The BaCaBO3F wide band gap host and the charge-compensated phosphors BaCaBO3F:Ln3+ (Ln = Ce, Tb, Gd) and BaCaBO3F:Ce3+,Tb3+ have been synthesized by a solid-state reaction method at high temperature. Their spectroscopic properties in the vacuum ultraviolet (VUV)–vis range have been investigated. The band gap of the host lattice is estimated at 7.8 eV, which is considerably higher than the value found from its diffuse reflectance spectrum. The 5d crystal field level locations, Stokes shift, and Huang–Rhys factor have been determined from the VUV excitation and near-UV emission spectra of BaCaBO3F:Ce3+. The 5d1 decay lifetime is 29 ns at x ∼ 0.005. Concentration quenching occurs for Ba1–2xCexNaxCaBO3F with maximum intensity at x ∼ 0.035 and with critical distance Rc = 17.7 Å. An unusual broad, intense band in the excitation spectrum of BaCaBO3F:Gd3+ is assigned to a near-defect exciton. Very weak emission from 5D3, and intense green emission from 5D4, has been assigned for BaCaBO3F:Tb3+. In the VUV region, the spin-allowed and the spin-forbidden 4f8 → 4f75d transitions are observed. The codoped system BaCaBO3F:Ce3+,Tb3+ exhibits emission color tunability when varying the excitation wavelength or the dopant ion concentration. The energy transfer from Ce3+ to Tb3+ takes place in the fast migration regime with kET ∼ 106–107 s–1 and with energy-transfer efficiency up to 23% for the samples investigated. The mechanism is revealed from energy level and decay measurements. This codoped system exhibits absorption bands near the major Xe discharge wavelengths.

Temperature-dependent luminescent property of Pr3+-doped NaGdTiO4 is investigated systematically. Because of the intensive quenching of 3P0 emission, a red emission of 1D2–3H4 transition is observed in NaGdTiO4:Pr3+ at room temperature (RT). At 77 K, a broad band emission of host at about 475 nm (blue) is observed. Interestingly, the intensity of the red emission is increased with increasing temperature from 77 to 350 K and keeps nearly stable to 400 K under the host excitation, whereas a decrease of the blue host emission is presented. The results show that the increased red emission of 1D2 is associated with temperature-assisted energy transfer from host to Pr3+. Because the relative intensity of the 1D2 and host emission is sensitive to temperature, a large variation of color is also observed from 77 K to RT.

A series of Ca3La3(1–x)Ce3x(BO3)5 phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed using the powder X-ray diffraction (XRD) data, which shows occupation of Ce3+ on both Ca2+ and La3+ sites with a preferred location on the La3+ site over the Ca2+ site. The prepared samples contain minor second phase LaBO3 with contents of ∼0.64–3.27 wt % from the Rietveld analysis. LaBO3:1%Ce3+ was prepared as a single phase material and its excitation and emission bands were determined for identifying the influence of impurity LaBO3:Ce3+ luminescence on the spectra of the Ca3La3(1–x)Ce3x(BO3)5 samples. The luminescence properties of Ca3La3(1–x)Ce3x(BO3)5 samples under vacuum ultraviolet (VUV) and UV excitation were investigated, which exhibited two-center luminescence of Ce3+, assigned to the Ce(1)3+ center in the La3+ site and Ce(2)3+ center in the Ca2+ site, taking into account the spectroscopic properties and the Rietveld refinement results. The influences of the doping concentration and the excitation wavelength on the luminescence of Ce3+ in Ca3La3(1–x)Ce3x(BO3)5 are discussed together with the decay characteristics.

A series of Na1+xCa1−2xCexPO4 phosphors were prepared by a high-temperature solid-state reaction method. Excitation spectra in VUV–UV range, emission spectra under VUV–UV and X-ray excitation, and luminescence decays were investigated for samples with different doping concentrations at RT and 14 K. Seven excitation bands and two emission bands due to f–d transitions of Ce3+ were found in the spectra. The observed spectroscopic properties may relate to multi-site occupancies of Ce3+ in NaCaPO4 but Ce3+ ions at different sites emit at similar wavelengths or the emission only occurs from the lowest energy site due to the efficient energy transfer between different Ce3+ sites.Highlights► Luminescence of Na1+xCa1−2xCexPO4 under VUV–UV and X-ray excitation was investigated. ► Seven excitation bands and two emission bands due to f–d transitions of Ce3+ were observed. ► Ce3+ ions may occupy multi-sites but they emit at similar wavelengths or only from the lowest energy site.

GdAl3(BO3)4:Eu3+ red-emitting phosphors were prepared by evaporation of nitrates and H3BO3 solution and then calcination at 1423 K. The excitation and emission spectra of undoped GdAl3(BO3)4 and Eu3+-doped GdAl3(BO3)4:Eu3+ phosphors in VUV–vis spectral range were investigated, showing that GdAl3(BO3)4:Eu3+ phosphors have better CIE chromaticity coordinates in comparison with the commercial phosphor (Y,Gd)BO3:Eu3+ (YGB). The energy transfer between Gd3+ and Eu3+ in Gd1−xEuxAl3(BO3)4 phosphors is discussed based on the luminescence spectra and the decay curves, demonstrating that the energy transfer of Eu3+ → Gd3+ is inefficient but that of Gd3+ → Eu3+ is efficient and the Gd3+ → Eu3+ energy transfer efficiency increases with the increasing of Eu3+ concentration.Highlights► The luminescence spectra of undoped GdAl3(BO3)4 and Eu3+-doped GdAl3(BO3)4:Eu3+ in the VUV–vis range and the luminescence decays were investigated. ► The excitation spectra of GdAl3(BO3)4:Eu3+ demonstrated that Eu3+ cannot transfer energy to Gd3+. ► The emission spectra and decay curves revealed that Gd3+ → Eu3+ energy transfer efficiency increases with the increasing of Eu3+ concentration.

The work presents the VUV-UV–vis photoluminescence of Ba3La(PO4)3:Ln3+ (Ln = Tb, Eu) prepared by a high-temperature solid-state reaction route. The excitation and emission spectra, the fluorescence decays, and the time-resolved emission spectra were measured and discussed in detail. The results reveal that Tb3+ can efficiently transfer excitation energy to Eu3+ via its 5d and 4f states and therefore sensitizes Eu3+ emission under VUV-UV excitation, resulting in tunable emission in a large color gamut.

This report presents the luminescence properties of Ce3+ and Pr3+ activated Sr2Mg(BO3)2 under VUV–UV and X-ray excitation. The five excitation bands of crystal field split 5d states are observed at about 46 729, 44 643, 41 667, 38 314 and 29 762 cm−1 (i.e. 214, 224, 240, 261 and 336 nm) for Ce3+ in the host lattice. The doublet Ce3+ 5d→4f emission bands were found at about 25 840 and 24 096 cm−1 (387 and 415 nm). The influence of doping concentration and temperature on the emission characteristics and the decay time of Ce3+ in Sr2Mg(BO3)2 were investigated. For Pr3+ doped samples, the lowest 5d excitation band was observed at about 42017 cm−1 (238 nm), a dominant band at around 35714 cm−1 (280 nm) and two shoulder bands were seen in the emission spectra. The excitation and emission spectra of Ce3+ and Pr3+ were compared and discussed. The X-ray excited luminescence studies show that the light yields are ∼3200±230 and ∼1400±100 photons/MeV of absorbed X-ray energy for the samples Sr1.86Ce0.07Na0.07Mg(BO3)2 and Sr1.82Pr0.09Na0.09Mg(BO3)2 at RT, respectively.

The spectroscopic properties of Na3Gd(PO4)2 and Na3Gd(PO4)2:Ce3+ phosphors in the VUV–UV spectral range were investigated. Five excitation bands of Ce3+ ions at Gd3+ sites are observed at wavelengths of 205, 246, 260, 292, and 321 nm. Doublet Ce3+ 5d → 4f emission bands are observed at 341 and 365 nm with a decay constant τ1/e around 26 ns. The X-ray excited luminescence of Na3Gd0.99Ce0.01(PO4)2 at room temperature shows a photon yield of ∼17,000 photons/MeV of absorbed X-ray energy.Research highlights► The spectroscopic properties of phosphors Na3Gd(PO4)2:Ce3+ in the VUV–UV range were investigated. ► Though there are different Gd3+ lattice sites, Ce3+ ions preferably enter one site. ► It was found that the phosphors show a reasonable light yield under X-ray excitation.

The VUV−vis spectroscopic properties of Tb3+ activated fluoro-apatite phosphors Ca6Ln2−xTbxNa2(PO4)6F2 (Ln = Gd, La) were studied. The results show that phosphors Ca6Gd2−xTbxNa2(PO4)6F2 with Gd3+ ions as sensitizers have intense absorption in the VUV range. The emission color of both phosphors can be tuned from blue to green by changing the doping concentration of Tb3+ under 172 nm excitation. The visible quantum cutting (QC) via cross relaxation between Tb3+ ions was observed in cases with and without Gd3+. Though QC can be realized in phosphors Ca6La2−xTbxNa2(PO4)6F2, we found that Gd3+-containg phosphors have a higher QC efficiency, confirming that the Gd3+ ion indeed plays an important role during the quantum cutting process. In addition, the energy transfer process from Gd3+ to Tb3+ as well as 5D3−5D4 cross relaxation was investigated and discussed in terms of luminescence spectra and decay curves.

This work presents the vacuum ultraviolet−visible spectroscopic properties of Tb3+ in Li(Y, Gd)(PO3)4:Tb3+ phosphors prepared by a high-temperature solid-state reaction. Under vacuum ultraviolet light excitation, tunable emission from the greenish-blue to yellowish-green region was obtained by changing the doping concentration of Tb3+. The visible quantum cutting via cross-relaxation between Tb3+ ions was observed and confirmed in the high Tb3+ concentration sample. In addition, the energy-transfer process from Gd3+ to Tb3+ as well as 5D3−5D4 cross-relaxation was also investigated and discussed in term of luminescence spectra and decay curves.

Rare-earth phosphates MGd(PO3)4:1.0 mol% Ce3+ (M = Li, Na, K, Cs) powder samples were prepared by a solid-state reaction technique at high temperature. The radioluminescence spectra and light-yield characteristic of MGd(PO3)4:Ce3+ under X-ray irradiation were determined. It was found that, from LiGd(PO3)4:Ce3+ to CsGd(PO3)4:Ce3+, with the increasing of M+ radius, the doublet emission energy of Ce3+ ions decrease gradually but the light-yield increase significantly. Especially, CsGd(PO3)4:1.0 mol% Ce3+ has the highest X-ray excited light-yield of 24,400 photons/MeV with maximal emission peaks at 337 nm and 358 nm at room temperature. Due to its suitable emission wavelength range, high light-yield, high chemical stability and fast luminescence decay of Ce3+ emission, CsGd(PO3)4:Ce3+ may be a promising scintillation material.

A series of Eu3+-activated tetra-molybdate phosphors BaGd2−xEux(MoO4)4 have been prepared by a solid-state reaction route. The photoluminescent properties of the phosphors were measured at room temperature. Photoluminescence of the phosphors under near UV excitation was enhanced by partial substitution of Mo6+ with W6+ or by co-doping other trivalent ions in BaGd2(MoO4)4:Eu3+. The luminescence of BaGd2[(Mo,W)O4]4:Eu3+ and BaGd2(MoO4)4:Eu3+,A3+ (A3+ = Y3+, Yb3+, and Lu3+) were compared with that of a conventional red phosphor Y2O2S:0.05Eu3+. The results show that sample BaGd1.2Eu0.8(WO4)0.4(MoO4)3.6 exhibits the strongest red emission under near-UV excitation. An intense red light-emitting diode (LED) was fabricated by combining this phosphor with a ∼395 nm-emitting InGaN chip, and the good performance of the LED demonstrates that the phosphor may be a suitable red component for application in near-UV InGaN chip-based white-light-emitting diodes.

A series of samples La2−xPrxCaB10O19 were prepared by a solid-state reaction technique. The spectroscopic characteristics, including the steady-state excitation and emission spectra in the VUV–vis range, the luminescence decays and the concentration quenching are investigated. Two types of emission, photon cascade emission (PCE) and f–d emission were observed in the samples. The phenomena are interpreted in terms of different occupancies for Pr3+ in La3+ and Ca2+ sites.

Eu3+-activated phosphors, Sr9R2−xEuxW4O24 (R=Gd and Y ), were prepared by the conventional solid-state reaction method and their photoluminescent properties were studied. The phosphors show intense red emission under 395 and 465 nm light excitation, which is matched with the light-emitting wavelength of a near-UV-emitting and a blue-emitting InGaN chips, respectively. Bright red-light-emitting diodes (LEDs) and white-light-emitting diodes (WLEDs) were fabricated by coating Sr9Y 0.4Eu1.6W4O24 phosphor onto ∼395 nm-emitting InGaN chips and ∼460 nm blue-emitting InGaN chips, respectively. The good performances of the LEDs demonstrate that the tungstates are suitable for application of near-UV and blue InGaN-based WLEDs.

The phosphors, NaLa1−xTbx(MoO4)2 and NaLa1−xTbx(WO4)2, were prepared by the solid-state reaction. The structures and photoluminescent properties of the phosphors were studied. The result of XRD patterns indicates that these phosphors are of the similar scheelite-like (CaWO4) isostructure. The absence of concentration quenching of Tb3+ was observed in NaLa1−xTbx(WO4)2 by investigating their photoluminescent properties. This phenomenon has not been reported before, as we known. The phosphor NaTb(WO4)2 shows stronger green emission under 378 nm excitation with broader excitation band, compared with NaLa1−xTbx(MoO4)2.

Sr3−2xCexNaxGaO4F phosphors were prepared through a high-temperature solid-state reaction method. Their photoluminescence and cathodoluminescence properties were investigated. Under ultraviolet light and low-voltage electron beam excitation, the phosphors show a typical broad emission of Ce3+ and the luminescence color can be tuned in a large gamut from greenish-blue to yellowish-green by changing Ce3+ concentration in the range of 0.01−0.30 (x value). Their luminescence behaviors are discussed in terms of Ce3+ in two distinct sites. The lowest 5d absorption bands are respectively at ∼405 and ∼430 nm for Ce(1)3+ [i.e., Ce3+ in Sr(1)2+ site] and Ce(2)3+ [i.e., Ce3+ in Sr(2)2+ site] centers, while the emissions from relaxed 5d state to 2F5/2 ground state are respectively at ∼456 nm for Ce(1)3+ and ∼567 nm for Ce(2)3+.

To obtain a suitable green phosphor for plasma display panels (PDPs), the samples of trivalent terbium-activated polyphosphate NaGd(PO3)4:Tb3+ (NGP:Tb3+) were prepared by a solid-state reaction technique at high temperature. The vacuum ultraviolet (VUV)–visible spectroscopic properties and luminescence decay characteristic were investigated. Because the phosphor NaGd1−xTbx(PO3)4 for x = 0.80 shows broad and strong absorption in VUV region, exhibits intensive emission under 147/172 nm excitation in comparison with the commercial green PDP phosphor Zn2SiO4:Mn2+, it is considered to be a new promising green phosphor for PDPs application.The vacuum ultraviolet (VUV)–visible spectroscopic properties and luminescence decay characteristic of the phosphor NaGd1−xTbx(PO3)4 were investigated.

A series of samples with nominal chemical formulas La2−xCexCaB10O19 and La2Ca1−2xCexNaxB10O19 were prepared by a solid state reaction route at high temperature. Their luminescence properties were investigated by the steady state excitation and emission spectra in the VUV−vis range, the luminescence decays, and the time-resolved emission spectra (TRES). The results demonstrate that Ce3+ ions occupy two lattice sites in all samples. The lowest 5d absorption bands for two sites are at about 272 (La3+ site) and 312 nm (Ca2+ site), respectively. The emission for Ce3+ in the La3+ site shows a shorter decay time of 12 ns, and the doublet emission bands have maxima at about 291 and 310 nm. The emission for the Ce3+ in Ca2+ site has a longer lifetime of 26 ns with band maxima at about 329 and 355 nm. Efficient energy transfer between both sites occurs in the samples.

The VUV−vis spectroscopic properties of Gd3+ in Na(Y,Gd)FPO4 are reported. The photon cascade emission was observed under 8S7/2 → 6GJ excitation in which 6GJ → 6PJ, 6IJ emissions of Gd3+ in the orange/red and near-IR range are followed by ultraviolet emission of 6PJ → 8S7/2 transitions. We demonstrate systematically the emission decays and concentration-quenching characteristics of 6GJ → 6PJ and 6PJ → 8S7/2 transitions under excitation into 6GJ levels. The effect of temperature on the 6GJ → 6PJ emissions is also presented.

Cerium-activated polyphosphates MGd(PO3)4 (M = Li, Na, K, Cs), were synthesized by a solid-state reaction technique at high temperature. The ultraviolet (UV) and vacuum ultraviolet (VUV) luminescence spectra of this series of samples were determined at room temperature (RT). On the basis of the VUV excitation spectra of MGd(PO3)4:1.0 at% Ce3+, the centroid shift εc, total crystal field splitting εcfs, and total redshift D(A) were calculated. According to the ligand polarization model, the values of the spectroscopic polarizability αsp were also calculated from the observed centroid shifts. It was found that the εc, εcfs, D(A) and αsp were influenced by the crystal structure. Based on the luminescence spectra of these polyphosphate samples, the energy level diagram of Gd3+–Ce3+ in MGd(PO3)4:Ce3+ systems was also proposed. Through the energy level diagram and the decay curves of Ce3+ emission, the energy transfer between Gd3+ and Ce3+ ions was investigated. It was found that there existed efficient energy transfer between Gd3+ and Ce3+ ions in MGd(PO3)4:Ce3+ systems. Considered the efficient energy transfer from Gd3+ to Ce3+ ions and short lifetime of Ce3+ emission, this series of compounds doped with Ce3+ ions could be used as promising scintillator materials.

Cerium-activated polyphosphates NaGd(PO3)4:Ce3+ were prepared by high temperature solid-state reaction technique. The emission spectra of NaGd(PO3)4:Ce3+ under UV and X-ray excitation as well as the fluorescence decay curves were determined. The phosphors show broad emission bands in the wavelength range 300–350 nm. The fluorescence decay of Ce3+ upon 297 nm excitation exhibits a single exponential characteristic with a decay constant around 12.4 ns. The light yield of NaGd(PO3)4:10.0 at% Ce3+ excited with X-rays at RT was calculated to be about 21 000 ± 1500 photons/MeV. Due to its suitable emission wavelength range, high light yield, shorter decay time and non-hygroscopic property, NaGd(PO3)4:Ce3+ may be a potential scintillator material.X-ray excited emission spectrum of NaGd(PO3)4:10.0 at% Ce3+ at RT. The dotted line is the X-ray excited emission spectrum of BaF2 as a reference.

A novel phosphor, NaEu(MoO4)2, co-doped with Bi3+ ions and Sm3+ ions was prepared by solid-state reaction technique, and its photoluminescent properties were investigated. The introducing of Bi3+ ions and Sm3+ ions broadened the excitation band of the phosphor NaEu(MoO4)2 and enhanced the emission intensity of Eu3+ under 395/405 nm light excitation. The emission of the phosphor shows very good CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates (x = 0.66, y = 0.34). The phosphor may be applied in fabrication of phosphor-converted light-emitting diodes (LEDs).

The phosphors NaGdFPO4:Ln3+ and GdPO4:Ln3+ (for Ln3+=Ce3+ and Tb3+) were prepared by solid-state reaction technique, the VUV–vis spectroscopic properties of the phosphors were investigated, and we vividly compare the luminescence of Ce3+ and Tb3+ in the hosts. For phosphors GdPO4:Ln3+, the band near 155 nm in VUV excitation spectrum is assumed to be the host-related absorption, and for NaGdFPO4:Ln3+ the absorption is moved to longer wavelength, near 170 nm, showing the P–O bond covalency increased after fluoridation. The f–d transitions of Ce3+ and Tb3+ in the host lattices are assigned and corroborated, and it was found that the 5d states are with lower energy in NaGdFPO4:Ln3+ than those in GdPO4:Ln3+. For fluoridation of GdPO4:Ln3+ to NaGdFPO4:Ln3+, the energy change of Ln3+ (Ln=Ce, Tb) 5d states is consistent with that of host-related absorption.The VUV excitation (a, under emission at 312 nm) and VUV-excited emission (b, excitation under 155 nm) spectra for phosphor GdPO4 at 293 K, the VUV excitation (c, under emission at 311 nm) and VUV-excited emission (d, excitation under 166 nm) spectra for phosphor NaGdFPO4 at 293 K.

•The energies of five crystal field split 5d states of Ce3+ in Ba2MgSi2O7 were determined by synchrotron radiation VUV-UV excitation spectrum.•The concentration effect, thermal stability of Ce3+ were investigated.•The energy transfer from Ce3+ to Eu2+ and its influence on luminescence decays of Ce3+ and Eu2+ were studied.A series of Ce3+ doped and Ce3+-Eu2+ co-doped Ba2MgSi2O7 phosphors was prepared via a high-temperature solid-state reaction technique. The photoluminescence properties, which include synchrotron radiation VUV-UV excitation spectra, emission spectra and concentration effect, thermal stability of Ce3+ are investigated. Hence the energies of the crystal field split 5d excited states of Ce3+ are determined. Due to spectral overlap, the energy transfer from sensitizer Ce3+ to activator Eu2+ in Ba2MgSi2O7:Ce3+, Eu2+ occurs, and the mechanism is demonstrated to be an electric dipole−dipole interaction.

Eu3+-activated phosphors, Sr9R2−xEuxW4O24 (R=Gd and Y ), were prepared by the conventional solid-state reaction method and their photoluminescent properties were studied. The phosphors show intense red emission under 395 and 465 nm light excitation, which is matched with the light-emitting wavelength of a near-UV-emitting and a blue-emitting InGaN chips, respectively. Bright red-light-emitting diodes (LEDs) and white-light-emitting diodes (WLEDs) were fabricated by coating Sr9Y 0.4Eu1.6W4O24 phosphor onto ∼395 nm-emitting InGaN chips and ∼460 nm blue-emitting InGaN chips, respectively. The good performances of the LEDs demonstrate that the tungstates are suitable for application of near-UV and blue InGaN-based WLEDs.

Eu3+-activated La2CaB10O19 phosphors with nominal chemical formulae La2−xEuxCaB10O19 and La2Ca1−2xEuxNaxB10O19 were prepared. The luminescence properties under VUV-UV excitation were investigated. Luminescence spectra reveal Eu3+ ions occupy both Ca2+ and La3+ sites in all samples. The O2− → Eu3+ charge transfer bands (CTBs) and the 5D0 → 7F0 emissions of Eu3+ at Ca 2+ site and La3+ site are identified, respectively. The schematic energy levels for Eu3+ in two sites were achieved. Luminescence decays of the 5D0 → 7F0 transitions were measured for both sites.

Photoluminescence properties of Ba2MgSi2O7:Eu2+ synthesized by a solid-state reaction method are first investigated in the vacuum ultraviolet (VUV) to visible (vis) excitation energy range. The band gap of the host is found to be around 7.44 eV. The incorporation of Eu2+ leads to bright green luminescence with weak thermal quenching above room temperature. Cathodoluminescence (CL) properties under low-voltage excitations are then studied, and the results suggest a potential application of the compound in field emission displays (FEDs). Electronic properties of the compound are finally calculated by a hybrid density functional theory (DFT) method, and are discussed in association with observed luminescence properties.

BaCa2MgSi2O8:Eu2+ phosphors have been prepared by a conventional high temperature solid state reaction technique. The emission and excitation spectra as well as the luminescence decays were investigated, showing that Eu2+ ions enter both Ba2+ and Ca2+ sites in the host. The structural refinement reveals that about 70% Eu2+ ions occupy Ba2+ sites in samples Ba1−xEuxCa2MgSi2O8 and BaCa2(1−x)Eu2xMgSi2O8. By investigation of the thermal-quenching, the schematic energy levels for Eu2+ in BaCa2MgSi2O8 were proposed. Intense blue emission was observed under 147, 172 and 254 nm excitation in comparison with the commercial blue phosphor BaMgAl10O17:Eu2+ (BAM), demonstrating the potential application of the phosphors in plasma display panels (PDPs) and tri-color fluorescent tubes.

Cerium-activated polyphosphates MGd(PO3)4 (M = Li, Na, K, Cs), were synthesized by a solid-state reaction technique at high temperature. The ultraviolet (UV) and vacuum ultraviolet (VUV) luminescence spectra of this series of samples were determined at room temperature (RT). On the basis of the VUV excitation spectra of MGd(PO3)4:1.0 at% Ce3+, the centroid shift εc, total crystal field splitting εcfs, and total redshift D(A) were calculated. According to the ligand polarization model, the values of the spectroscopic polarizability αsp were also calculated from the observed centroid shifts. It was found that the εc, εcfs, D(A) and αsp were influenced by the crystal structure. Based on the luminescence spectra of these polyphosphate samples, the energy level diagram of Gd3+–Ce3+ in MGd(PO3)4:Ce3+ systems was also proposed. Through the energy level diagram and the decay curves of Ce3+ emission, the energy transfer between Gd3+ and Ce3+ ions was investigated. It was found that there existed efficient energy transfer between Gd3+ and Ce3+ ions in MGd(PO3)4:Ce3+ systems. Considered the efficient energy transfer from Gd3+ to Ce3+ ions and short lifetime of Ce3+ emission, this series of compounds doped with Ce3+ ions could be used as promising scintillator materials.

The potential application of Sr8(Si4O12)Cl8:Eu2+ phosphor in wide gamut 3D-PDP and 3D-FED is demonstrated in consideration of its high emission intensity (∼148% and ∼280% of commercial BAM) under VUV light and low-voltage electron beam excitation, suitable chromaticity coordinates (0.136, 0.298) and short decay time (∼0.9 μs).

Ultraviolet and vacuum ultraviolet spectra are reported for Ce3+ and Eu2+ singly doped into the Sr5(PO4)3Cl lattice, synthesized by a solid-state process. The maxima of the emission intensities are at 346 nm (Ce3+) and 442 nm (Eu2+), with lifetimes of 25 ns and 494 ns, respectively. The Ce3+ excitation spectra exhibit five bands assigned to vibronic structures of the 4f1 → 5d1 electronic transition, in addition to the host absorption at 166 nm at 295 K. The 5d centroid shift is similar to that in SrCl2:Ce3+. The Eu2+ excitation spectrum is mainly comprised of two 4f7 → 4f65d broad bands between 200 nm and 450 nm, peaking at 343 nm and 275 nm. Both of the singly doped systems exhibit concentration quenching, with energy transfer rates being in the low (μs)−1 range for concentrations up to 4 at.% of total cations. The energy transfer rates are linearly and quadratically related to dopant ion concentrations of Ce3+ and Eu2+, respectively. The energy transfer between Ce3+ and Eu2+ in the co-doped Sr5(PO4)3Cl lattice has been studied by the analysis of intensity and decay measurements of both Ce3+ and Eu2+ emissions. The electric dipole–electric dipole transfer has an efficiency of 91% in Sr4.8−xCe0.01EuxNa0.01(PO4)3Cl, with a critical distance of 21 Å. Although energy transfer between Ce3+ and Eu2+ upon excitation into the overlapping Ce3+/Eu2+ absorption band is definitely demonstrated by the dramatic shortening of Ce3+ lifetime, the emission of Eu2+ is also enhanced at low Ce3+ concentrations using exclusive excitation into the Eu2+ absorption band. The quantum yields (QY) of the co-doped system approach those of BaMgAl10O17:Eu2+ (BAM) for the excitation wavelengths of 317 and 365 nm, but are inferior for 254 nm excitation. The addition of 1 at.% Ce3+ to Sr4.98Eu0.02(PO4)3Cl increases the QY by ∼20% of the original value. The cathodoluminesce of the co-doped phosphor is comparable to that of BAM, with CIE coordinates of the emission (0.274, 0.237), so that the application as a field emission display phosphor is proposed.

0.5% Ce3+ doped BaCa2MgSi2O8 phosphor was prepared by a conventional solid state reaction method. Luminescence spectra as well as fluorescence decay were monitored in the VUV-UV range. Ce3+ emissions are assigned to cerium ions on a Ba2+ site, and the five 4f–5d excitation bands of Ce3+ were determined at low temperature. The light yield is estimated to be around 10600 ph MeV−1 under X-ray excitation. X-ray absorption near-edge structure (XANES) was explored to study the energy transfer efficiency to optical centers from each element in the phosphor; the results show that the contributions to luminescence are not identical for each element.

A series of LiCaBO3:Ce3+/Pr3+ phosphors were prepared by a high-temperature solid state reaction method. A structure refinement for the LiCaBO3 compound was performed based on powder X-ray diffraction (XRD) data. The energies of the crystal field split excited 5d states of Ce3+ and Pr3+ in LiCaBO3 were determined from synchrotron radiation VUV-UV excitation spectra. Furthermore, the influence of the doping concentration and temperature on the emission properties of Ce3+ was investigated, and the vacuum referred binding energy (VRBE) scheme for all lanthanide 4f and 5d states in LiCaBO3 was constructed to estimate the thermal activation energy of Ce3+ luminescence quenching. The Pr3+ to Ce3+ energy transfer (ET) and its influence on luminescence decays of Pr3+ and Ce3+ were studied in detail. Finally, the X-ray excited luminescence spectra were measured to evaluate the possible X-ray detection applications.